The present invention relates to lost foam countergravity casting of metals and alloys.
Countergravity casting processes for making investment castings in ceramic molds are described in U.S. Pat. No. 3 900 064 and 5 069 271. The patents involve using wax patterns of the castings to be made in the well known lost wax process to make a gas permeable investment shell mold by the well known lost wax technique. The casting processes include vacuuming molten metal upwardly into a vertical central sprue of the gas permeable mold from an underlying molten metal pool with the sprue sized to permit the castings to solidify while the molten metal in the sprue remains unsolidified for return to the pool for reuse in manufacture of further castings, thereby reducing cost of manufacture.
The well known lost foam casting process involves pouring molten metal into a vaporizable, foamed plastic pattern surrounded by an unbonded foundry sand support media in a container. The molten metal destroys and replaces the plastic pattern before the unbonded sand collapses and produces a casting having the shape of the pattern. The lost foam process was extended by U.S. Pat. No 4 874 029 to provide for countergravity casting of much thinner castings at lower cost than achievable by pouring molten metal into foam plastic patterns. In these lost foam processes, the container for the patterns and the foundry sand support media is vibrated, or the foundry sand is optionally fluidized in the container, during filling with sand so as to distribute the sand to all pattern surfaces. However, the vibratory or fluidization forces exerted on the foam plastic patterns can mechanically damage them, especially in the event fragile foam plastic patterns having one or more thin pattern walls are used.
An object of the present invention is to provide a countergravity casting process and apparatus for making cast components using vaporizable patterns in a manner that provides investing of the patterns in a particulate media, such as unbonded foundry sand, while avoiding the need for vibration or fluidization of the particulates.
Another object of the present invention is to provide a countergravity casting process and apparatus for making cast components in a manner that provides faster and more complete replacement of the patterns with molten metal.
Still another object of the present invention is to provide a countergravity casting process and apparatus for making cast components using foam plastic patterns in a manner that permits reuse of molten metal in a sprue.
In an illustrative method embodiment of the present invention, the patterns of the components to be cast are placed in a container, and refractory particulates are introduced into the container about the patterns. After filling of the container with particulates, the container is evacuated and rotated in a manner to cause movement of the particulates to any regions of the thin wall patterns not yet invested or supported by the particulates. Container rotation can occur about an axis normal to a container longitudinal axis and optionally about another axis, which may include the container longitudinal axis. The container and its contents then are ready for countergravity casting of molten metal into the patterns from a source of molten metal to destroy and replace them in the particulate media.
In accordance with another method embodiment, as the molten metal advances upwardly progressively destroying and replacing the pattern assembly, thermal decomposition vapors of the pattern material are vented through a vent passage in a sprue interconnecting the patterns in a manner that permits faster and more complete replacement of the patterns with the molten metal. After at least partial solidification of the castings, the container typically is moved to disengage from the molten metal source and permit molten metal residing in the sprue to drain by gravity to the source for reuse.
In another illustrative embodiment of the present invention, a vaporizable pattern assembly is formed having a sprue connected to a plurality of thin wall patterns of articles to be cast. The pattern assembly is supported in a refractory particulate media in an open bottom container that can be evacuated to provide subambient pressure therein. The patterns preferably comprise a relatively dense foam plastic pattern material that imparts increased strength to the thin wall patterns. The sprue includes a vent passage that communicates to the particulate media to vent pattern thermal decomposition vapors to the particulate support bed in a manner that enables faster and more complete replacement of the patterns with molten metal in the support media, despite use of a relatively dense pattern material.
Advantages and objects of the present invention will be better understood from the following detailed description of the invention taken with the following drawings.